1. Field of the Invention
Structures and methods consistent with the present invention relate to a soldering, and more particularly, to a soldering structure comprising a Zn layer and a soldering method.
2. Description of the Related Art
Soldering, which is one of joining methods required for the electronic joints, joins two components together by melting a third material at the temperature below 450° C. The third material is called solder. Traditionally, a solder comprising lead (Pb) has been used for a long time. However, as it was recognized that lead is poisonous and its use has been regulated, a soldering method using a Pb-free solder is attracting great attention. In the conventional soldering method using a Pb-free solder, a Cu bonding layer is employed between a substrate and the Pb-free solder.
As Pb-free solders, a solder using Sn as the main component is on the rise. An example of a solder structure including a Sn solder is shown in FIG. 1.
The conventional Pb-free soldering structure of FIG. 1 includes a substrate 10, a bonding layer or bonding pad 20, and a solder 30. The bonding layer 20 employs Cu layer. The solder 30 may be made of SnAgCu.
The soldering structure as shown in FIG. 1 shows weak bonding strength and brittleness of the bonding interface, resulting in the decrease in the product reliability. That is, when the solder 30 including Sn is bonded to the bonding layer 20, Ag3Sn platelets and a Cu3Sn phase are generated at the interface of the solder 30 and the bonding layer 20. As a result, the soldering structure is prone to break.
After a reflow process is carried out in the conventional soldering process, the Sn-based solder undergoes an undercooling during the cooling process. The undercooling of the Sn solder causes a formation and separation of Ag3Sn particles, which flow into the undercooled molten Sn. Accordingly, the Pb-free solder is irregularly constituted. The Ag3Sn particles rapidly grow in the molten Sn and form Ag3Sn platelets.
In the soldering structure as described above, if cracks occur afterwards, the cracks rapidly spread along the interface on which Ag3Sn platelets are formed. An electronic device fabricated by bonding a chip onto the substrate using the conventional soldering structure as described above will become defective due to cracks which may formed when an impact is given to the device or the stress is concentrated to the soldered joints. Since the cracks quickly spread in the conventional soldering structure, the solidity of the soldering joint becomes deteriorated.
Meanwhile, in the conventional soldering structure, a thicker inter-metallic compound (IMC) layer may be formed, and thus easily causes a formation of voids.
FIG. 2 is a conceptual diagram of the IMC and the voids generated in the conventional soldering structure. As shown in FIG. 2, when the solder 300 reflows on the bonding layer 20, Cu6Sn5 phase is generated at the interface area between the bonding layer 20 and the solder 300. Afterwards, Cu3Sn phase is generated in the interface between Cu6Sn5 and Cu as aging process proceeds.
The compounds produced in the interface between the metals, such as Cu3Sn phase and Cu6Sn5 phase, are commonly called the IMC.
Since the thick IMC of the conventional soldering structure generates voids or pores in the internal interface, the soldering strength deteriorates. In specific, as the solid state diffusion takes place in the generation of the Cu3Sn phase, voids or pores may be generated in the interface. As a result, the soldering structure may be easily destroyed.
Therefore, the conventional soldering structure and method has difficulty in using the lead-free solder.